Not applicable.
1. Field of the Invention
This invention generally relates to an optical projection system comprising a light source, a mask holder, a projection lens system, and specifically relates to an optical projection system for photolithogaphy used in producing microstructured devices such as integrated circuits or other semiconductor devices. During the fabrication of such devices photolithography transfers an image from a photographic mask to a resultant pattern on a semiconductor wafer. Such photolithography generally includes alight exposure process, in which a semiconductor wafer is exposed to light having information of a mask pattern. Optical projection systems are used to perform the light exposure process.
In general, the transferred mask patterns are very fine, so that optical projection lens systems are required to have a high resolution. The high resolution necessitates a large numerical aperture of the optical projection lens system and also a nearly no aberration of the optical projection lens system in the light exposure field.
2. Related Prior Art
For example, some projection lens systems are proposed in the German Patent Application DE 198 18 444 A1. The shown projection lens system comprises 6 lens groups. The first, third, fifth and sixth lens groups have positive refractive power and the second and fourth lens groups have negative refractive power. To get a high resolution in all shown examples, aspherical surfaces are in the fourth and fifth lens groups.
Some purely refractive projection exposure objectives of microlithography are discussed in SPIE Vol. 237 (1980) page 310 ff. There are shown objectives of the planar style and the distagon style, wherein the new style of objective comprises two waists for setzval correction.
It is an object of this invention to provide a further excellent optical projection lens system for photolithography.
The optical projection lens system of the invention comprises in a direction of the light a first lens group having positive refractive power and a second lens group having negative refractive power and establishing a beam waist of minimal beam height. A further lens arrangement follows the second lens group. At least one lens, which is arranged before first beam waist, has an aspherical surface. Further, lenses comprising aspherical surfaces in all other groups will be helpful to reduce the needed amount of material and to reduce the length of the optical projection lens system.